The present invention relates to a process for producing a polyurethane foam. The invention further relates to a polyurethane foam obtainable by the process of the invention. These foams may in particular have a bimodal cell size distribution.
Theoretical considerations make nanocellular or nanoporous polymer foams particularly good materials for thermal insulation. The interior dimensions of these foam structures are in the region of the mean free path lengths of a gas molecule. The contribution of the gas to heat transmission can thus be reduced. Polyurethanes are a polymer group often used in thermal insulation.
When polyurethane foams are produced, a polyol component, which also comprises a blowing agent, is reacted with an isocyanate. The reaction of isocyanate with water produces carbon dioxide, which also acts as blowing agent.
The decisive step for the formation of the foam and therefore for the subsequent cell size of the hardened foam, is the nucleation provided by blowing agents, since each cell in the foam has been produced from a gas bubble. A relevant observation here is that after nucleation no new gas bubbles are generally produced, but instead blowing agent diffuses into existing gas bubbles.
Addition of stabilizers promotes the emulsification of the various components, influences nucleation, and inhibits coalescence of the expanding gas bubbles. They also influence cell opening. In open-cell foams, the membranes of the expanding pores are opened and the pore walls are retained.
One possible approach emulsifies a supercritical blowing agent in the reaction mixture and then hardens the foam after a pressure reduction. A known variant here is the POSME process (principle of supercritical micro emulsion expansion). The blowing agent in the said process takes the form of a microemulsion. Microemulsions form under particular conditions, which depend inter alia on the concentration of the emulsifiers and on the temperature. A feature of microemulsions is that they are stable and that the non-polar phase, the blowing agent in this case, can be present in the form of very small droplets within the polar phase. The diameters of these droplets can be in the range from 1 to 100 nanometres.
DE 102 60 815 A1 discloses foamed material and a production process for the foamed material. The intention is that foamed material with nano-size foam bubbles be produced without any need to surmount the energy barrier which usually arises at phase transitions and in nucleation processes. An objective associated with this is controllable production of a foamed material which has a numeric density of foam bubbles of from 1012 to 1018 per cm3, and also an average diameter of foam bubbles of from 10 nm to 10 μm. It is based on the dispersion of a second fluid in the form of pools within a matrix of a first fluid. The first fluid is present in the form of matrix in a reaction space, and the second fluid is present in the form of pools. The second fluid is converted into a near-critical or supercritical state with a density close to that of a liquid, through a change in pressure and/or temperature. The second fluid is therefore entirely or almost entirely in the form of pools which have uniform distribution within the entire first fluid. Depressurization causes the second fluid to revert to a state with gaseous density, and the pools here are expanded to give nanometre-size foam bubbles. There is no need to surmount any energy barrier, and there is no requirement that the blowing agent molecules diffuse to the expanding bubbles.
A polymerizable substance is generally proposed as first fluid here. However, express mention is made only of acrylamide, which polymerizes to give polyacrylamide, and melamine, which polymerizes to give melamine resin. The second fluid should be one selected from a group of hydrocarbon substances, such as methane or ethane, or else from alkanols, fluorochlorocarbons or CO2. An amphiphilic material is also used, and this should have at least one block with affinity for the first fluid, and at least one block with affinity for the second fluid.
WO 2007/094780 A1 discloses, in relation to polyurethane foams, a resin composition with a polyol, an ethoxylated/propoxylated surfactant initiated by a short-chain compound, and also a hydrocarbon as blowing agent. The ethoxylated/propoxylated surfactant increases the solubility and/or compatibility of the hydrocarbon blowing agent and improves the phase stability of the resin composition. The resin composition is suitable for the reaction with polyfunctional organic isocyanates to produce cellular polyurethane foams and cellular polyisocyanurate foams.
The surfactants are obtained through the reaction of ethylene oxide and propylene oxide with an initiator from the group of compounds having an alkylene-oxide-active hydrogen atom and a C1 to C6 aliphatic or alicyclic hydrocarbon group, compounds having an alkylene-oxide-active hydrogen atom and a C6 to C10 aryl or alkylaryl hydrocarbon group, or combinations thereof. The initiator is preferably selected from the group of the C1 to C6 aliphatic or alicyclic alcohols, phenol, C1 to C4 alkylphenols and combinations thereof.
Butanol-initiated propylene oxide/ethylene oxide surfactant is mentioned as an example. As an alternative, the surfactant can also comprise an alkoxylated triglyceride adduct or an ethoxylated derivative of a sorbitan ester. The blowing agent can be a C4 to C7 aliphatic hydrocarbon, C4 to C7 cycloaliphatic hydrocarbon or a combination thereof. Pentanes are mentioned as an example.
However, the examples mentioned do not disclose any polyol composition in which the selection of the surfactants leads to the presence of the blowing agent in the form of a microemulsion.
Specific siloxane surfactants are addressed inter alia in US 2005/0131090 A1. Here, a process is disclosed for producing rigid polyurethane foams through reaction of a polyisocyanate and of a polyol in the presence of a urethanization catalyst, of a blowing agent, and optionally of water and of a silicone surfactant. Blowing agents used are C4- or C5-hydrocarbons, or a mixture of these. The average molar mass of the blowing agents is ≦72 g/mol and their boiling point is in the range from 27.8 to 50° C. The silicone surfactant comprises a polyether-polysiloxane copolymer which is represented by the following general formula: (CH3)3—Si—O— (Si(CH3)2—O)x—(Si(CH3)(R)O)y—Si(CH3)3, in which:
R=(CH2)3—O—(—CH2—CH2—O)a—(CH2—CH(CH3)—O)b—R″ and in which R″ is H, (CH2)zCH3 or C(O)CH3. Furthermore: x+y+2 is 60-130, x/y is 5-14 and z is 0-4. The total molar mass of the surfactant, based on the above formula, is from 7000 to 30 000 g/mol. The proportion by weight of the siloxane in the surfactant is from 32 to 70% by weight, the average molar mass (BAMW, blend average molecular weight) of the polyether fraction is from 450 to 1000 g/mol, and the content of ethylene oxide, expressed in mol %, in the polyether fraction is from 70 to 100 mol %. However, the said publication does not relate to any microemulsions or blowing agents in the supercritical state. Instead, the siloxane surfactant is used as cell stabilizer.
GB 2 365 013 A discloses alkylene-oxide-modified silicone glycols for stable polyester polyol compositions. A polyester polyol composition comprises a phthalic anhydride-initiated polyester polyol, a C4-C6-hydrocarbon blowing agent and an alkylene-modified silicone glycol compatibilizer with an HLB value of from about 5 to about 8. The blowing agent is soluble in the polyol composition, and the risk associated with blowing agents of this type in the production of rigid polymer foam products is thus reduced. Rigid foams are provided with good dimensional stability and with improved insulation properties. An isocyanate-modified silicone glycol compatibilizer is also disclosed.
The said Patent Application states that in some instances a particular blowing agent forms a microemulsion with the polyol and with other components. However, there is no disclosure as to whether supercritical conditions prevail here for the blowing agent. Instead, the information about microemulsions relates to the test for determining the storage stability of the polyol compositions. In the said test, the polyol composition and the blowing agent are mixed in a glass jar with cap and are shaken, and are stored at room temperature for five days. If no phase separation occurs, the blowing agent is found to be soluble in the polyol composition and the composition is found to be stable in storage. However, storage in a glass jar with cap at room temperature is not likely to provide any conditions under which a C4-C6-hydrocarbon is present in the supercritical state.
The said Patent Application moreover mentions that, during the production of the foams, the starting materials can be introduced at a temperature of from 15° C. to 90° C., preferably from 20° C. to 35° C., into an open or closed mould. The prevailing pressure can be above atmospheric pressure. The mixing of the isocyanate with the polyol composition which comprises dissolved blowing agent can be achieved through stirring or at high pressure through injection. The temperature of the mould can be from 20° C. to 110° C., preferably from 30° C. to 60° C. and in particular from 45° C. to 50° C. Here again, there are no indications that supercritical conditions for the blowing agent prevail.
WO 2001/98389 A1 describes the rapid depressurization of CO2-containing reaction mixtures. That Patent Application relates to a process for producing polyurethane block foam where a reactive polyurethane mixture comprising carbon dioxide is depressurized suddenly from a pressure above the equilibrium solution pressure of the carbon dioxide to atmospheric pressure. The reactive liquid polyurethane mixture is foamed with release of dissolved carbon dioxide, and the foamed mixture is applied to a substrate and then hardened to give the block foam. The carbon dioxide is initially completely dissolved in the reactive mixture or in at least one of the components, polyol and isocyanate, at a pressure substantially above the equilibrium solution pressure. The pressure is then reduced to a pressure close to the equilibrium solution pressure, whereupon at some junctures here the pressure is less than the equilibrium solution pressure, with release of small amounts of the carbon dioxide with formation of a microdispersion of bubbles, the components are mixed if appropriate, and the pressure is suddenly reduced to atmospheric pressure, before the carbon dioxide released is completely redissolved. However, that document gives no indications of nanocellular foams or supercritical conditions for the blowing agent.
A foam with a multimodal cell size distribution (multimodal foam) provides performance advantages, for example greater toughness and improved insulation capability, in comparison with a conventional foam of identical polymer constitution which however has a generally uniform cell size distribution. A foam with a bimodal cell size distribution (bimodal foam) is a type of multimodal foam.
Processes described in the past for producing multimodal foams expand a foamable polymer composition which comprises water. Water has a tendency to produce corrosive acid when it reacts with halogenated flame retardants. The corrosive acid is undesirable because it can corrode the process apparatus. WO 2002/034823 A1 describes a process which can produce a multimodal foam and which requires no water and preferably an environmentally compatible blowing agent.
That Patent Application relates to a process for producing a multimodal thermoplastic polymer foam comprising the following sequential steps: (a) dispersing a blowing agent stabilizer and a blowing agent into a heat-plasticized thermoplastic polymer resin at an initial pressure to form a foamable composition, and (b) expanding the said foamable polymer composition in the substantial absence of water and at a pressure less than the said initial pressure to produce a multimodal thermoplastic foam.
A disadvantage here, however, is that only heat-plastifiable thermoplastics can be processed. The said process excludes, for example, thermoset polyurethane polymers. It would be desirable, however, to have processes which can produce polyurethane foams with in particular bimodal cell size distribution with use of supercritical blowing agents to achieve small cell sizes.